Leading human rights group Amnesty International said Monday that it fears a humanitarian catastrophe is brewing in Darfur. Sudan is reported to have begun sending troops and arms into Darfur, claiming that it wants to stop violence in the war-torn region.

Amnesty International and other observers fear that the build-up of Sudanese Armed Forces and weapons in Darfur is the precursor to a ground offensive.

Sudan is under pressure to allow a United Nations force to replace an African Union mission that has struggled with funding problems and a weak mandate.

Sudan has fiercely resisted calls for a United Nations force and has, instead, proposed sending 10,000 of its own troops to Darfur to stop violence in the region.

Mudawi Ibrahim heads the largest Sudanese aid organization working in Darfur .

Ibrahim returned from Darfur this morning and says the buildup of troops is making Darfuris "very nervous."

"The problem is that the Darfurians have seen the Sudanese army as an enemy," he explained. " How can you make them feel safe with such an army? I think to strengthen the African Union is the way. But you can't prepare yourself to have all these troops on the ground and if the African Union pulls out you have only the Sudanese army. This will make a problem."

Ibrahim says that he believes Sudan is preparing for an offensive against groups who have failed to accept the Darfur Peace Agreement.

Only one faction of the Sudan Liberation Army, led by Minni Minnawi, signed onto the May 5 peace deal.

Two other rebel factions refused to sign the accord, claiming it did not offer Darfuris enough political power or enough compensation for victims of the three-year war.

In recent months the rebels have fought one another for control of the region. Civilians have borne the brunt of the conflict, which has hindered humanitarian aid and forced thousands of Darfuris to flee their villages.

"The way the Sudanese government wants to fix it is to eliminate those who have not signed the agreement," he said. "They want to crush the rebel movements who have not signed. Fighting with them, eliminating them, crushing them, literally. They are protecting civilians the way they see it. They see the non-signatories are the ones that are making the insecurities, so eliminate the ones who have not signed, you bring security."

The United Nations Security Council is set to vote Monday on a draft resolution that recommends sending around 20,000 U.N. troops to Darfur.

The United States dispatched Assistant Secretary of State Jendayi Frazer to Khartoum this week, in an attempt to coax Sudan into accepting a UN force.